1) Field of the Invention
This invention relates to improvements in an air bag for use in an air bag system in which the air bag is promptly inflated in the event of a collision of an automotive vehicle or the like to protect an occupant from impact.
2) Description of the Related Art
As a cushioning system for providing additional safety beyond that provided by a seat belt system for holding safely an occupant or the like of a vehicle, an air bag system has been put into practical use. With the air bag system, the body of an occupant, especially his head and chest, is supported by a balloon-like air bag in the event of a collision and the internal gas is then gradually released, whereby a cushioning stroke is prolonged to reduce impact to the body.
Such an air bag system includes a module constructed of a center pad, a folded air bag, an inflator (gas generator) and the like. The module can be built into a steering wheel, for example. When triggered by an electric signal from a sensor which has detected a vehicular collision, the inflator is activated so that the air bag is instantaneously inflated by the resulting gas. Upon activation of the air bag system, a top wall of the center pad is split apart by the expanding pressure of the air bag, thus allowing the air bag to come out through the resulting opening and to be inflated substantially above the center pad. When the module of the air bag system is arranged inside an instrument panel, activation of the air bag system either tears apart or opens an air bag cover by the resulting expansion pressure of the air bag, thus permitting substantial inflation of the air bag.
Conventional air bags have each been fabricated by coating a woven fabric, which is formed of synthetic fibers such as nylon 66 fibers, nylon 6 fibers, polyester fibers or aramid fibers, with synthetic rubber, cutting the resulting, coated base fabric in a predetermined pattern to obtain cut fabric pieces and then sewing the cut fabric pieces along the peripheries thereof by a sewing machine.
In all such conventional air bags, the woven fabric is coated over its entire surfaces with a synthetic rubber composed of a chloroprene copolymer, a silicone rubber or the like so as to avoid the potential danger that, due to leakage of gas from the inflated air bag, the inertial energy of an occupant is not sufficiently absorbed and the occupant hence collides against a steering wheel or a dashboard. This has, however, led to the problem that the air bag so fabricated is heavy and stiff and its fabrication cost is high.
An air bag system making use of a special air bag has therefore been proposed. To fabricate the special air bag, a woven fabric with a high set to reduce gas permeation therethrough is employed. This woven fabric is cut without coating the woven fabric with a synthetic rubber such as described above. The cut fabric pieces thus prepared are then sewn into the special air bag.
The preparation of cut fabric pieces by cutting a fabric not provided with any synthetic rubber coating, in a predetermined pattern as described above, however, results in the problem that the cut fabric pieces may fray where cut, such as at the outer peripheries of the cut fabric pieces and the inner peripheries of openings and/or holes thereof. Such fraying leads to reduced strength adjacent to the cuts, so that sewn parts in the outer peripheries of the cut fabric pieces sewn by a sewing machine as described above undergo seam slippage by abrupt inflation of the resulting air bag. Fraying may therefore lead to gas leakage and/or fabric rupture. Such gas leakage causes a problem in that the air bag fails to fully absorb inertial kinetic energy of an occupant in the event of a vehicular collision.
Upon fabrication of an air bag by sewing a plurality of cut fabric pieces, the sewing must be conducted without risk of any appreciable reduction in strength by seam slippage or deviation of stitches from the correct stitch line. Such sewing requires substantial cost, leading to high fabrication cost.